The Australian Transport Safety Bureau (ATSB) is Australia's national transport safety investigator. The ATSB's function is to improve safety and public confidence in the aviation, marine and rail modes of transport. The ATSB is Australia's prime agency for the independent investigation of civil aviation, rail and maritime accidents, incidents and safety deficiencies.

Summary

Summary

On the morning of 11 June 2008, a Bell 412 helicopter,
registered VH-UAH, was being used to conduct training operations
from Wollongong Aerodrome, New South Wales. Shortly after landing
on the runway, the helicopter developed severe vertical airframe
vibrations that resulted in reduced pilot control. In an attempt to
mitigate the vibrations, the pilot raised the helicopter into the
hover, however, the vibrations continued to increase in severity.
In response, the pilot lowered the collective to set the helicopter
back down onto the runway. The resulting heavy landing caused
serious damage to the helicopter, but the crew were not
injured.

A subsequent examination of the helicopter's flight control
system revealed an anomaly with the collective hydraulic actuator.
Excessive free play was found to have developed in the bolted joint
between the pivot bolt and the pilot input lever, which then
allowed vertical vibrations and controllability issues to develop.
It is likely that free play at the bolted joint was introduced when
the collective actuator was last overhauled.

As a result of this occurrence, the collective actuator
manufacturer revised the tensioning procedures and requirements for
the pivot bolt assembly during the overhaul process. In addition,
the helicopter operator changed its inspection regime of the
collective servo-hydraulic actuator units in its fleet of Bell 412
helicopters and issued a 'flight staff instruction' to provide
guidance to pilots on what actions to take if they experienced
unusual or excessive vibrations during flight.